The way I see it, yeah the pokemon would gain "stats" less quickly because they would earn less EVs (Effort Values or points awarded for defeating a certain pokemon Ex. beating pikachu gives 2 points in speed and 4 points give you 1+ in your overall speed stat, the maximum amount in one stat can be 255 points and the total max is 510) since it has to defeat less pokemon to level up with its boosted exp amount. However, in the end it if it is trained all the way and earned all if its EVs, it would level out to a nonboosted exp pokemon's stats.
Hope that was somewhat helpful or someone else can clarify. xD